Single chamber rate responsive pacemakers have been available in the pacing community for about a decade now. Since their introduction, rate responsive pacemakers have gained increased acceptance as providing an improved response to the patient's physiological need. Atrial-based pacers, i.e., atrial synchronous or atrial sequential pacers, were probably the first form of rate responsive pacemaker. However, such atrial-based pacemakers, including DDD pacemakers, are dual chamber devices that require a second lead to be positioned in the atrium, and further are not satisfactory for patients with certain cardiac conditions. A number of indicators other than the atrial rate have been suggested as suitable for determining the optimal pacing rate in patients where DDD pacing is not indicated. The first commercially available form of single chamber rate responsive pacemaker was the QT driven rate responsive pacemaker, marketed by the assignee of this invention. See U.S. Pat. No. 4,228,803, to Rickards. Subsequently, other pacemakers utilizing other monitored variables have been introduced commercially, the best known example being the Activitrax model manufactured by Medtronic, Inc.
Even though there are now pacemakers utilizing a number of respective different monitored variables for determining rate, the general structure of the rate responsive pacemaker is common and generic. Any rate responsive pacemaker incorporates a sensor adapted to sense a patient's variable which indicates, or reflects the patient's needs at any given moment, and which provides both quantitative and progressive information. For example, in the QT pacemaker, the sensor is the same lead that delivers the pacing pulses. Following delivery of a pacing pulse, it senses the stimulus-evoked T-wave, and provides an electrical signal which can be compared in the time domain to the stimulus pulse, thus providing the QT time interval information. Whatever sensor is used for sensing an indication of physiological demand, in order to be useful in a modern implantable pacemaker, it must have long term stability and reliability.
The general structure of a rate responsive pacemaker also includes a means for carrying out an algorithm by which the pacemaker determines the indicated pacing rate on the basis of information delivered by the sensor. The function of the algorithm is to provide a correspondence, or correlation between the sensed indicator variable and the pacing rate, so that the pacemaker is accurately responsive to the monitored information.
Lastly, of course, a standard multi-programmable single-chamber pacemaker is utilized. For purposes of illustrating the preferred embodiment, this specification references the Vitatron TX pacemaker (QT driven). See U.S. Pat. Nos. 4,228,803; 4,305,396; 4,644,954; and 4,527,568, which are incorporated herein by reference. See also the publication Vitatext, October, 1987, published by Vitatron Medical B.V.
Using the QT driven rate responsive pacemaker as an example, the algorithm determines the change in pacing rate per ms change in the stimulus-T interval, which is defined as the slope. Correct programming of the slope, and accurate processing by the pacemaker are seen to be crucial factors in obtaining optimal pacemaker functions. Initially, the slope, or relationship between QT interval and pacing rate, was fixed. Recent investigations into the relationship between the QT interval and the desired pacing rate have indicated that the slope should be variable, i.e., there should be means for automatic adaptation of the slope as a function of patient history, in order to maintain optimal long-term rate response, even under changing circumstances. This finding reflects a long felt need of rate responsive pacemakers, namely that the correlation data between the monitored patient variable or variables and the desired pacing rate generally should be dynamic and subject to variation from patient to patient and as a function of patient history. Accordingly, there is a need in rate responsive pacemakers for implementation of automatic adaptation of the slope, or a pacing rate/indicator variable characteristic.